Design and performance of an aerodynamic molecular beam and beam detection system

Design and performance of aerodynamic molecular beam syste

Heavy-Fermions in a Transition-Metal Compound: LiV2O4LiV_2O_4

The recent discovery of heavy-Fermion properties in Lithium Vanadate and the enormous difference in its properties from the properties of Lithium Titanate as well as of the manganite compounds raise some puzzling questions about strongly correlated Fermions. These are disscussed as well as a solution to the puzzles provided.Comment: late

Staggered Flux Phase in a Model of Strongly Correlated Electrons

We present numerical evidence for the existence of a staggered flux (SF) phase in the half-filled two-leg t-U-V-J ladder, with true long-range order in the counter-circulating currents. The density-matrix renormalization-group (DMRG) / finite-size scaling approach, generalized to describe complex-valued Hamiltonians and wavefunctions, is employed. The SF phase exhibits robust currents at intermediate values of the interaction strength.Comment: Version to appear in Phys. Rev. Let

Fibrous model for the simulation of the cyclic behaviour of 3D reinforced concrete frames

Relatório 07-DEC/E-04Non-linear finite element analysis has attracted a lot of attention in last few decades, as real life problems dealing with analysis of concrete beams and columns are generally non-linear in nature. This project deals with non-linear analysis of beam and columns, using Timoshenko beam theory. The Timoshenko beam theory is applied to every element, but before applying the theory the element is divided into fibers. The fibers as a whole will govern the behavior of element. The section of each element is divided in “finite elements”. Each finite element is a fiber along the length of element. Each fiber element is independent in terms of the material constitutive laws, which govern the behavior, while at the same time they are associated to each other by the relation of displacements. The material laws of each fiber deals with the stress-strain relation The numerical results are compared with the experimental results, which show a reasonable correlation, and in turn demonstrate the capability of the proposed model to depict the cyclic behavior of concrete elements

Design-curves of strain softening and strain hardening fibre reinforced concrete elements subjected to axial load and bending moments

Several structural components made by fibre reinforced concrete (FRC) are submitted to axial load and bending moments. In some cases fibres cannot replace completely the conventional reinforcement, even if strain hardening FRC is used; therefore the optimization of the reinforcement solution for these elements depends on the post-cracking behaviour of the FRC, as well as the percentage of conventional reinforcement to replace. To fully exploit the FRC capabilities, the development of a suitable, comprehensive and design-oriented model of its tensile response is of the utmost importance. To provide a practical tool for the pre-design of FRC-structural-elements subjected to axial load and bending moments, design curves were generated using a computational program capable of simulating the main features of the tensile and compressive behaviour of strain softening and strain hardening FRC’s, and steel bars. The post-cracking tensile behaviour of the FRC is characterized by two parameters that define its residual strength ratio (α) and its corresponding tensile strain ratio (β). To generate these curves, a parametric study is carried out based on the tensile parameters α and β and considering distinct mechanical reinforcement ratios, from 0 to 1 with a step of 0.2. For every reinforcement ratio, four distinct cases in terms of β are considered; additionally, for every case of β, α is divided from strain hardening to strain softening in five distinct residual strength classes. By sequentially varying α and β, the bending capacity of a section is evaluated to provide a design perspective of the effect of ductility and strength. The model used in this study is described, the design curves are presented and analyzed and some practical design examples are provided

Numerical model for CFRP confined concrete elements subject to monotonic and cyclic loadings

Uniaxial cyclic and monotonic compression tests were carried out on partially and fully wrapped concrete cylinders with Carbon Fibre Reinforced Polymer (CFRP) wet lay-up sheets. The influence of the concrete compressive strength, CFRP stiffness, geometric confinement arrangement and loading type on the compressive behaviour of reinforced concrete column elements of circular cross section up to their failure was assessed. A uniaxial stress-strain constitutive model is proposed, and the results obtained from the experimental tests were used to calibrate some of the parameters of this model, and to appraise the model performance. This model allows the simulation of reinforced concrete members by using Timoshenko one-dimensional elements, in the context of the finite element method (fibre model). Good agreement was obtained between numerical simulations and experimental results for both monotonic and cyclic loading tests.The first author acknowledges the support provided by the grant within the ambit of the research program PABERPRO supported by Program POCI 2010 - IDEIA, Project No. 13-05-04-FDR-00007, contract reference ADI/2007/V4.1/0049. The present work was also supported by Program PIDDAC, Project No. PTDC/ECM/74337/2006 by FCT. The authors acknowledge the materials generously supplied by S&P and Degussa Portugal

Cost competitive steel fibre reinforced self-compacting concrete for structural applications

The present work resumes the research carried out to develop a cost competitive Steel Fibre Reinforced Self-Compacting Concrete (SFRSCC) for structural applications, such is the case of lightweight panels for building facades. A mix design method was developed taking into account the influence that fibres introduce in the skeleton phase of the composition. The concrete precast industry is one of the most suited industries for the use of SFRSCC, due to the possibility of assuring high levels of quality control from the mix composition phase up to the curing process of the manufactured structural elements. Since demoulding of the precast elements should be executed as earliest as possible, due to economic and stock management reasons, the influence of age on the SFRSCC mechanical properties was assessed carrying out compression and flexural tests. Compression and monotonic cyclic tests were also carried out to assess the predictive performance of analytical formulations. To appraise the structural possibilities of the developed SFRSCC, panel prototypes were subject to flexural and punching load configurations. The obtained results showed the great benefits provided by the fibre reinforcement mechanisms. Finally, a real scale panel was constructed and tested in an industrial environment, in order to help in the establishment of the best practices of technology transfer from laboratory to factory, as well as to determine the load carrying capacity of this type of structural elements.(undefined

Singular Effects of Impurities near the Ferromagnetic Quantum-Critical Point

Systematic theoretical results for the effects of a dilute concentration of magnetic impurities on the thermodynamic and transport properties in the region around the quantum critical point of a ferromagnetic transition are obtained. In the quasi-classical regime, the dynamical spin fluctuations enhance the Kondo temperature. This energy scale decreases rapidly in the quantum fluctuation regime, where the properties are those of a line of critical points of the multichannel Kondo problem with the number of channels increasing as the critical point is approached, except at unattainably low temperatures where a single channel wins out.Comment: 4 pages, 2 figure

Enhanced Impurity Scattering due to Quantum Critical Fluctuations

It is shown on the basis of the lowest order perturbation expansion with respect to critical fluctuations that the critical fluctuations give rise to an enhancement of the potential scattering of non-magnetic impurities. This qualitatively accounts for the enhancement of the resistivity due to impurities which has been observed in variety of systems near the quantum critical point, while the higher order processes happen to give much larger enhancement as seen from the Ward identity arguments. The cases with dynamical critical exponent $z$=2 and $z$=3 are discussed explicitly.Comment: Submitted to J. Phys. Soc. Jpn. on 27 September, 200

New excitations in bcc 4^{4}He - an inelastic neutron scattering study

We report neutron scattering measurements on bcc solid $^{4}$% He. We studied the phonon branches and the recently discovered ''optic-like'' branch along the main crystalline directions. In addition, we discovered another, dispersionless "optic-like'' branch at an energy around 1 meV ($\sim$~11K). The properties of the two "optic-like" branches seem different. Since one expects only 3 acoustic phonon branches in a monoatomic cubic crystal, these new branches must represent different type of excitations. One possible interpretation involves localized excitations unique to a quantum solid.Comment: 4 pages, 3 figures, accepted by PRB, Rapid Communication